1. Field of the Invention
The present invention relates to a keyboard switch detect and assignor system useful in a keyboard musical instrument.
2. Related Applications
This invention is related to the inventors' copending U.S. patent application Ser. No. 603,776 filed on Aug. 11, 1975 entitled POLYPHONIC TONE SYNTHESIZER.
3. Description of the Prior Art
In an electronic keyboard instrument of the digital tone generation variety, a significant cost arises if a tone generating system were available for each switch on a keyboard as is customarily used for analog tone generation systems. Therefore in digital tone generation systems such as DIGITAL ORGAN described in U.S. Pat. No. 3,515,792, COMPUTOR ORGAN described in U.S. Pat. No. 3,809,786, and POLYPHONIC TONE SYNTHESIZER described in copending U.S. patent application Ser. No. 603,776 the number of tone generation subsystems is advantageously 12 while the number of keyboard switches may be 147. For low cost implementation a means must be incorporated for assigning 12 tone generators on demand to 147 keyboard switches.
Another problem arising in the economical construction of commercially viable digital tone generators is the desire to use batch fabricated, digital microelectronic techniques. Typically, such microelectronic device packages have about 40 leads which is not consistent with the 147 switches whose states must be transferred to these devices.
One approach to the problem of the limited number of leads available on a microelectronics device package is to use time division multiplexing of the key states of the musical instrument. With this technique a unique time slot is irrevocably assigned to each keyboard switch. The state of each such switch is indicated by the presence or absence of a signal pulse in the time slot assigned to the switch. The advantage of time division multiplexing is that the entire status of all the instrument's keyboard switches can be transmitted on a single signal lead. A drawback of time division multiplexing is that the scanning or search time for the instrument is fixed and is independent of the number of switches that have been closed. This fixed time can be undesirable because the wasted time used to sequentially scan an entire array of switches can lead to loss of key state detection when the musician plays very fast.
A time division multiplex note selection is shown by Watson in the U.S. Pat. No. 3,610,799. There all the keyboard switch state information is combined into unique time slots on a single multiplex line. The total scanning time required is K t, where K is the number of switches and t is the time slot allotted to each switch.
Klann disclosed a time division multiplexing system in U.S. Pat. No. 3,614,287 which includes provision for intermanual coupling. By pulse controlled sequential connection of manual coupling and keyboard switches an economy in wiring is obtained and keys actuated on one keyboard can cause voices to sound which are associated with the same or another keyboard.
Pearson, in U.S. Pat. No. 2,989,885 discloses a system for commutating separate waveform generator outputs onto a single line for subsequent processing by a single waveform shaper and sound system. There, delay line commutation, at a rate which is high in comparison to the frequency of the generated tones, is used to mix the outputs of key-switch selected waveform generators onto a common line. Pearson's technique allows the use of common tone generation circuits but requires a separate line from each waveform generator to the associated keyboard switch.
Deutsch and Griffith, in U.S. Pat. No. 3,899,951 disclose a key switch scanning and encoding system for a musical instrument in which open switches may be scanned at a faster rate than closed switches. A coded signal is created for each detected closed switch. The key switches are arranged in a matrix of M groups, each connected to N common output lines. The switch groups are enabled sequentially, one at a time. As each group of switches is enabled, the N output lines sequentially are gated to the coding matrix. If the gated line is associated with a closed switch in the enabled group, an output code is produced by the code matrix, which, together with a signal designating the enabled group, uniquely identifies the closed switch. If an open switch is scanned, no code is produced by the code matrix. This no-code condition immediately causes the next switch matrix output line to be gated to the code matrix. In this manner, open switches are "skipped over" or scanned at a rapid rate.
An object of the present invention is to provide a system for detecting the change of state of key switches in a keyboard musical instrument and causing a number of tone generation systems to be assigned or unassigned in response to detected key state changes. Economy in switch scanning is achieved by scanning groups of switches successively and interrupting group scanning only when a change in a key switch state is detected thereby effecting an average decrease in total scanning time. Wiring economy is achieved without the time rigidity inherent with time division multiplexing.